1. Field of the Invention
The present invention relates to binder compositions and processes for producing them, and particularly to a binder for chemically resistant concrete and a process to produce this binder. Most advantageously the invention can be used for manufacturing finishing blocks and panels, bearing structural members intended for use in a chemically agressive enviroment.
Though the chemical industry in most countries began to develop intensively relatively long ago, the problem of making not expensive chemically resistant concrete has not been adequately solved until the present time. It is a common knowledge that the concrete physical and chemical properties and its cost depend on the properties and characteristics of the binder and the process for producing it. Therefore, most attempts in solving this problem have been aimed at improving binder compositions and processes for producing them.
2. Description of the Prior Art
There is known a process for making concrete, using as a binder a silicic acid hydrosol (FRG Patent No. 1,240,457) and wherein to a solid silicate of the alkaline element with a silica modulus (a molar ratio of SiO.sub.2 to an alkali metal oxide) from 1 to 4.2 is added either a mixture of silicic acid hydrosol with a water-insoluble inorganic silicate in an amount from 0.3 to 3% by weight, or one of insoluble colloid metal oxides such as Al.sub.2 O.sub.3, TiO.sub.2 ZnO.sub.2, SnO.sub.2, CrO.sub.3, F.sub.2 O.sub.3 having particle size less than 200.
Japan Patent Specification No. 38-10178 describes a method for making an acid-resisting cement which is a dried and powdered alkaline hydrosilicate with a silica modulus not higher than 5, whereto are added additional agents for neutralizing alkali in the alkaline hydrosilicate to produce hydrated silica serving as a binding agent, and thereby improving water resistance of the cement.
However, in all the above cases as the binding agent is used a hydrated silica which is the most chemically unstable modification of SiO.sub.2, and therefore the resultant binders being sufficiently stable in concentrated acids, exhibit low stability in water, aqueous solutions of salts and weak solutions of acids, especially hot ones, and in alkalt solutions and vapours. In addition, since the above binders harden mainly due to dehydration of hydrosilica when the latter is dried, pores and capillaries, which form as a result of drying, bring down the binder strength, water resistance and impermeability in solutions, vapours and gases.
U.S. Pat. No. 3,498,802 describes a steam treatment process for producing thermoplastic materials and hydraulic cements, which comprises mixing materials, oxides and other ingredients which when melted together produce alkali silicate glass containing about 80-94 mol percent SiO.sub.2 and 6-20 mol percent R.sub.2 O, wherein R.sub.2 O consists of Na.sub.2 O, K.sub.2 O, and mixtures thereof, distributed throughout the volume of glass, ballmilling said mixture and melting it at a temperature of about 1500.degree., cooling the resultant melt to a glass, grinding the glass thus obtained to a particle size less than 0.149 mm (preferably less than 0.074 mm), and partially crystallizing anhydrous powdered glass by heating at about 900.degree.-1000.degree. C. to produce cristobalite, quartz and tridymite.
The resultant anhydrous semicrystalline powder containing some quantity of alkali silicate glass is used as an initial material for making thermoplastic materials and hydraulic cements. This powder, when treated in gaseous environment of at least 50% by weight steam at a pressure of at least one atmosphere and a temperature between about 100.degree.-200.degree. C. for a period of time sufficient to develop at least a surface portion on the particles of said powder, contains within its volume up to about 30% by weight of water.
The semicrystalline glass powder exhibits cementing properties due to adhesion between particles through silanol groups Si--OH when being dried. The presence of cristobalite, quartz or tridymite in the glass particles makes the resultant binder more durable, and decreasig oxides of the alkali metals in the volume of semicrystalline glass to 20-6 mol percent, i.e. increasing silica modulus from 4 to 15.7 permits the chemical stability of said binder to be improved in comparison with the afore-said cementing agents. The above (binder, however contains as a cementing agent a hydrated silica exhibiting insufficient chemical stability, and the presence in the hydrated semicrystalline glass volume of a rather considerable amount of alkali oxides and a large spesific surface area of the powdered particles affect the chemical resistance and mechanical strength of the hardened composition.
This is explained by that the hydrated amorphous cementing agent under the action of water, aqueous solutions and vapours is partially dissolved and washed out, and completely dissolved in alkali solutions.
The necessity of grinding the initial material and repeated heating of the alkali silic glass to a high temperature makes longer the production process and results in an elevated energy consumption.
There is also known a binder for making a chemically resistant concrete, featuring better physical and chemical properties (cf. U.S. Pat. No. 3,754,952). This binder consists of 40-80 weight percent finely divided quartz sand having a specific surface area of 1000-5000 cm.sup.2 /g, and 20-60 weight percent finely divided alkali silica ingredient, said quartz sand containing a silica component bound with at least one of oxides R.sub.2 O, wherein R is sodium, potassium. High silica glass containing within its volume from 8 to 13 mol percent sodium oxide or potassium oxide, is used as a silica component.
The process to produce this binder comprises the following steps: mixing quartz sand with at least one of such compounds as sodium carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide; heating the resultant mixture for producing and melting an oxide R.sub.2 O, wherein R is sodium, potassium; treating and cooling the resultant melt to produce alkali silica ingredient; grinding the resultant alkali silica ingredient; grinding quartz sand to obtain a specific surface area thereof from 1000 to 5000 cm.sup.2 /g; mixing a finely divided alkali silica ingredient with a finely divided quartz sand. According to this process at least one of said compounds (sodium carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide or mixtures thereof) is added to quartz sand in amounts of 10 to 30 weight percent.
To obtain a high-silica alkaline glass, a mixture of the quartz sand with one of said compounds is heated to a temperature of 1580.degree.-1800.degree. C. The mixture is treated at this temperature to completely melt down the quartz sand and produce a homogenous high-silica alkaline glass melt. When being cooled the resultant melt hardens and cracks to form granules with a size from 0.5 to 20 cm, which are then crushed. The binder thus produced represents a homogenous composition consisting of amorphous silica and potassium oxide and/or sodium oxide.
When the above binder is used for manufacturing concrete articles it is mixed with water and then treated in autoclaves in a saturated steam environment at a temperature not less than 190.degree. C. In this case the binder hardens due of vitreous silica dissolving and crystallizing on the finely divided particles of the quartz sand.
Thus, unlike the known binder compositions the above binder contains as a main cementing agent as anhydrous crystalline silica which is the most thermodynamically and chemically stable with a normal state of SiO.sub.2. This accounts for excellent physico-chemical properties of a quartz based binder and its chemical resistance not only in acid solutions by any concentration but also in alkali solutions with pH being up to 12, as well in salt solutions and organic dissolvents.
However, when the concrete made with the use of this binder is exposed for a long period of time to water environment, its strength lowers in the first two months by 40-50%. This is explained by the fact that the alkaline hydro-silicates contained in the binder dissolve in water and are washed out from the concrete, which results in a higher porosity lowering the strength thereof. Therefore, a higher water resistance of the concrete made with the use of the above binder can be achieved only be decreasing the content of sodium and potassium oxides in the binder. It has ben found however that even at maximum temperatures (up to 1600.degree. C.) which can be obtained in glass furnaces the content of said oxides cannot be lowered below 8 mol percent. At the same time using higher temperature obtainable in electric furnaces entails considerable consumption of electric energy, affects the durability of concrete and is not advantageous from a commercial point of view. In addition, grinding relatively durable glass granules after their being cooled requires a considerable amount of energy.